The ASHRAE Standard 55-1981 entitled "Thermal Environment Conditions for Human Occupancy" sets out the following parameters which require design attention:
Operative Temperature (typical ranges for a building in which occupants are mostly sedentary depend on humidity but span approximately 3.5.degree. C. within the global ranges, summer 22.degree. C.-27.degree. C., winter 20.degree. C.-23.degree. C.) PA0 Humidity (4.2-12 g/kg moisture ratio) PA0 Air movement (summer not exceeding 0.25 m/sec.), (winter not exceeding 0.15 m/sec.) PA0 Mean radiant temperature (operative temperature normally being an average of air temperature and mean radiant temperature) PA0 Thermal resistance of clothing PA0 Occupants' average metabolic rate (having regard to activity level).
A revision of this Standard, designated AINSI/ASHRAE Standard 55-81R has been released for public review and proposes tighter limits by specifying that the relative humidity should lie between 60% and 30% and narrowing the temperature range by approximately 0.5.degree. C.
This invention addresses all the above parameters, and, in addition, addresses the ventilation requirements which require a minimum air velocity through air distribution registers for proper diffusion of the supply air. It does not directly address other parameters listed in the Standard, such as non-steady and non-uniform temperature, radiant asymmetry and floor temperatures. It does, however, provide a means and method whereby operative temperature and the insulating effect of most people's clothing may be estimated, and whereby a conditioned space may be retained within that portion of the "comfort zone", illustrated for a specific example situation in the psychrometric chart on page 5 of the ASHRAE Standard, necessary to ensure also that the relative air velocity requirements, illustrated for example in FIG. 17 of Chapter 8 of ASHRAE Fundamentals 1985, are satisfied at all times.
The ability to vary the volume of the conditioned air supply to offset the sensible load in individual zones often causes the Variable Air Volume (VAV) system to be preferred to the Constant Air Volume (CAV) system, in which variations in sensible load are accommodated by changing the conditioned supply air temperature but maintaining its volume flow. Both systems suffer from imperfections and these become manifest as the load sensed by the control system reduces, that is, as the sensible load reduces. In VAV systems often the volume of ventilation air delivered to the minimum load zone is insufficient to avoid stuffiness; lack of air motion accentuates the sense of discomfort and dissatisfaction felt by the occupants. Also the humidity of the air can rise to unacceptable levels at part load. The CAV system avoids the stuffy, stagnant air complaints but frequently results in even less acceptable levels of humidity.
The invention is applicable to both existing and new VAV or CAV systems.
Reference can be made to Australian patents 530554 and 597757, and U.S. Pat. No. 4942740. These patents relate to some of a series of inventions for which patents have been granted are pending and which trace the development of several methods of air conditioning which when combined become the method known as the low face velocity/high coolant velocity (LFV/HCV) method. This invention embodies features of said patents, and relates to a means and method whereby the thermal conditions for human comfort can be yet more closely achieved, which is the principal purpose of this particular invention. As indicated above the method may be used with both constant air volume (CAV) and variable air volume (VAV) systems and is compatible with all conventionally employed coolants. To a limited degree the present method can be made compatible with conventional systems which are unrelated to the earlier inventions by the proponents but is most readily effected in conjunction with the invention of said patent 597757 and U.S. Pat. No. 4942740.
Physically based empirical equations have been developed to describe the thermal equilibrium between a human subject and the surroundings. The effects of each of the parameters discussed above on the rate of heat loss from the human subject are combined in an equation known as "the comfort equation". This long equation and its physical and empirical bases are succinctly summarized by B. W. Olesen in an article entitled "Thermal Comfort", Bruel & Kjaer Technical Review No. 2, 1982, and in more detail in standard texts. The physically based "comfort equation" allows the quantitative estimation of the various heat gains and losses by the subject but does not indicate the reaction of the subject to those gains and losses. Thermal comfort is defined as "that condition of mind in which satisfaction is expressed with the thermal environment". By testing the reactions of many hundreds of subjects to defined conditions within fully instrumented environmental test chambers, Professor P. O. Fanger of the Technical University of Denmark determined the most probable reactions of subjects and correlated these with the various effects on heat gains and losses embodied in the "comfort equation" . This he did in a manner which allows the most probable "predicted mean vote" (PMV) of persons to their thermal environment to be deduced through solution of the "comfort equation". Fanger's results are compatible with those of professor A. P. Gagge and others in the United States of America and have been verified and extended by researchers in many other countries. These results have been drawn together to form the basis for the ASHRAE Standard 55-81 on thermal environmental conditions for human occupancy. This Standard is advisory. It indicates the thermal conditions for which designers should aim in order to ensure that the majority of occupants feel thermally comfortable, i.e. not too hot, not too cold, not too moist, not too dry.
It is important to note that human comfort involves factors other than thermal comfort. Lighting level and colour, noise level and spectrum, posture, odour, touch, disturbance by breeze and by other persons can, if unacceptable, cause discomfort so nullifying attempts to satisfy conditions for thermal comfort to which the present invention specifically relates.
Numerous tables and charts have been constructed from the "comfort equation". No one single table or chart is sufficient to cover fully the influence of all the above listed variables. Nevertheless the major factors influencing human comfort are revealed by an examination of several of these charts. The aforesaid article by B. W. Olesen indicates that to illustrate all aspects of the "comfort equation" requires twenty eight different charts or diagrams.
The comfort equation expresses the energy balance between a person and their surroundings assuming that steady state equilibrium has been established. Using the notation of ASHRAE Fundamentals Handbook (1989) the total rate of energy output by the person in a steady state situation is equal to the metabolic rate. Some of this energy may be expended in performing mechanical work such as lifting a weight, as when walking up stairs, but the remainder appears as heat which must be lost to the surroundings if the person's basal temperature is to remain constant without the body invoking the thermoregulatory reactions of heavy sweating if too hot or shivering (to increase metabolic rate) if too cold. Thus the net rate of heat loss from the person per unit of skin surface area is (M-W) Watts per square meter.
The mechanisms by which the heat is lost are by transfer through the skin, Q.sub.sk, and by transfer through the lungs, that is by respiration, Q.sub.res.
The loss from the skin can be subdivided into a loss of sensible heat by convection, C, and by radiation, R, and a loss of latent heat through evaporation of moisture from the skin, E.sub.sk.
The loss by respiration is substantial. It can be divided into a convective loss C.sub.res and an evaporative loss E.sub.res.
All quantities are expressed in units of Watts per square meter of skin surface. When a "standard" body surface area, known as the "Dubois surface area", is specified the metabolic rate may, for ease of comparison, be expressed in the "met" unit where 1 met=58.2 W/m.sup.2 =50 Kcal/(h.m.sup.2) is the metabolic rate of a healthy adult person when seated quietly.
For a nude subject the surface area of skin can be determined and the skin temperature measured at representative points. Furthermore the heat transfer coefficients for convection and radiation, hence the sensible heat exchange with the surroundings, and the rate of evaporation of moisture from the skin can be determined. Similarly the sensible heat and the moisture losses from the lungs can be obtained from empirical equations deduced by Professor Fanger. Thus, all parameters of the comfort equation may be determined for the nude subject.
The effect of clothing is to add a layer of insulation to parts of the body. This insulation may be described as if it is a single equivalent uniform layer over the whole body. The insulating value is expressed in the units of "clo" where 1 clo=0.155 m.sup.2. .degree.C/W. The clothing also changes the surface area through which heat and moisture are exchanged with the surroundings and hence a small correction must be made to the Dubois surface area. The clo values for a wide range of garments from underwear to fur top coats have been tabulated in various reference books and are summarised in the aforesaid ASHRAE Standard.
Taking all factors into account P. O. Fanger in his book "Thermal Comfort", published in the readily available edition in 1982 by Krieger Publishing Company, Florida, developed the single equation which is the equation now most frequently referred to as "the comfort equation". The equation is written in the form given below. In the present invention ideally it is solved as an algorithm within the control system or, in the simplest realization, its solution is estimated from tabulated data for later combination with other data to set manually a zone thermostat.
The Fanger comfort equation is ##EQU1## and M=Metabolic energy production rate, W/m.sup.2 W=External work, W/m.sup.2
f.sub.cl =Ratio of surface area of clothed body to that of nude body PA1 t.sub.cl =Temperature of surface of clothing, .degree.C. PA1 E.sub.r =mean radiant temperature received by subject, .degree.C. PA1 h.sub.c =convective heat transfer coefficient W/m.sup.2 K. PA1 t.sub.a =air temperature in conditioned space, .degree.C. PA1 p.sub.a =partial pressure of water vapour in air, kPa PA1 k=0.155m.sup.2. .degree.C./(clo.W)=a unit conversion PA1 I.sub.cl =intrinsic clothing insulation.
The values of h.sub.c and f.sub.cl are given by ##EQU2## where V=relative velocity of air, m/s.
The difference between the left hand and right hand sides comfort equation is the thermal load on the body. The thermal load L is defined in ASHRAE 1989 Fundamentals Handbook as the difference between the internal heat production and the heat loss to the actual environment for a person hypothetically kept at comfortable skin temperatures and thermoregulatory sweat secretion rate for the actual activity level.
Fanger devised a voting scale for comfort and means of determining the predicted mean vote (PMV) of a large group of subjects for a given environment. The scale is
______________________________________ +3 hot +2 warm +1 slightly warm 0 neutral -1 slightly cool -2 cool -3 cold ______________________________________
The predicted mean vote was found to be fitted closely by the equation EQU PMV=[0.303 exp(-0.036 M)+0.028]L
where the thermal load L is determined from the comfort equation as indicated above.
The percentage of people dissatisfied with a given thermal environment may be related to the predicted mean vote and it has been found that not more than 10 percent of occupants will be dissatisfied, that is 90 percent will be satisfied, if EQU -0.5.ltoreq.PMV.ltoreq.+0.5.
These limits define the range of conditions within which the thermal environment is controlled according to the present invention. It may be noted that even for a predicted mean vote of zero, five percent of the occupants are likely to be dissatisfied.
It must be stressed that this is one only of the criteria available for determining acceptable thermal environmental conditions. We seek here to establish the method of achievement of human thermal comfort rather than the specific criteria used to measure that thermal comfort.
While most designers are successful in satisfying the thermal comfort criteria at peak load conditions, few if any have been able also to satisfy the criteria at all operating loads without resorting to the costly practice of overcooling and then reheating the air. This lack of success has caused many designers to ignore the recommendations of the aforesaid Standard. This in turn has contributed to the development of the "sick building syndrome". The problem stems from a fundamental incompatibility between the recommendations of the Standard and the means by which conventional air conditioning systems are controlled.
It is the aim of this invention to remove this incompatibility to allow the requirements for the thermal comfort of occupants to be satisfied at all conditions of operation of the air conditioning system. To do this the broad comfort zone depicted on the aforesaid ASHRAE psychrometric chart must be subdivided into a series of narrower bands each providing the "target" for operation over its own range of operating load conditions and occupant related characteristics.
The narrow "target zones" must embrace the wide range of clothing worn by occupants of an air conditioned space during the operating year, the diverse ranges of activity by the occupants varying from sedentary (met=1) to very active (met=3), and the need to consider relative air velocity (velocity of air over occupants of a conditioned space), air dry bulb temperature, radiant temperature and operative temperature, volume flow rate of air, sensible and total heat load, and humidity ratio. If these matters are considered, the level of human comfort now deemed desirable can be achieved only by adjusting from one narrow target zone to another such that effectively a narrow "moving comfort target zone" is defined within the relatively broad ASHRAE Standard comfort zone. This moving target zone will occupy different positions on a psychrometric, or psychrometric type, chart as both occupant related and system related conditions change during the operating year.
However, the Applicants herein have ascertained that under most climatic conditions the LFV/HCV air conditioning system, the subject of aforesaid U.S. Patent 4942740, can inherently restrain humidity in the occupied space from rising above the limit recommended by the aforesaid Standard. Control of relative air velocity, supply air dry bulb temperature and dehumidifier size can, in this invention, achieve a design condition within the required very narrow target zone within the general comfort zone. The location of the target zone itself may be "moved" on a psychrometric chart, manually or automatically (or a combination of both), by changing control set points to accommodate changes in occupant clothing or activity, changes in the level of direct solar or other thermal radiation and changes in ambient conditions. Provided the building design avoids excessive direct solar input through windows, diurnal adjustment is rarely required.